I'm posting this discussion to help those who are looking to finish off some gear sets in the remaining online time we have left. With sound strategy, every quest can be done smoothly, and even with just 4 or 5 people. However, I won't recommend going with fewer than 6 unless everybody knows their role well, and everybody knows how each other will act in given situations. The latter just comes from questing with a set group for a while, getting to know each other.
I'll edit things below based on how discussions go. I'm just throwing down a start for discussion points.
The single most important part to doing any quest: communication! Get a plan set before you even start a quest. In general it works best to divide enemies and kill them 1 at a time. Make sure everybody knows the order of attack, and who will do what tanking. While a strong spear in a group can tank everything, groups without a spear should plan on a tank (or 2 if the party cannot handle 2 enemies on them) and order of attack.
I'm going to say this quest is most difficult, because no matter what strategy is set, there's always a chance of the entire party being wiped out. The most effective strategy with this quest has involved 2 spears, where one will tank the little guys, and the other will tank the usurper. If you have 1 spear in the party, it may work best for them to tank the little guys while the party handles the usurper. If the little guys are focused on the spear, the rest of the party doesn't have to worry about them removing favors and can more easily focus on offense. Without any spears, then there should be 1 or 2 dedicated to the removal of the little guys, and support otherwise.
In terms of support, know that the first part of the battle is mostly magical attacks with only a couple physical, so be more concerned with mana barrier and angel's favor. In the 2nd part, the attacks are 100% physical so only worry about fortify and fates favor. Also, usurper's slow attack is particularly bad, a mage or bow should always be ready to counter with hasting.
This quest comes in 2nd, but for the wrong reason... that pathetic tag-along named Miu. At the beginning of the quest, whoever talks to miu should place a lifestone on her. At least 2 other party members should have lifestones, and the order of using them should be discussed in advance.
Everybody should have their best offense on hand, and focus on offense as much as possible. You only have 15 minutes to reach the final tree for your S rank. The main concern in battle is to make sure miu is protected. A couple people should focus on clearing the little stuff in each fight, with a couple more starting the onslaught on the boss. Ideally miu never draws attention, but be wary of those first couple boss attacks, and if she gets within range of their aoe attacks.
Having a bow or spear in the party makes the final boss fight easier to deal with, but given how small the fight area is, those wyvern aoe attacks will cover just about the entire area. The tank should draw whichever wyvern is decided into one corner, and the rest of the party keep the other in the opposite corner. Given the constant respawn of little stuff, 2 should be focused on clearing those when they appear. 1 (ideally a mage) should watch to restore miu's fates favor, and everybody watch for the need to use those lifestones!
In terms of support, they made one thing easy for us... absolutely nothing uses magical attacks. You only need to worry about fates and fortify in every boss fight. Every boss can hit hard (they have fmq bounty power for offense) so be ready for a bit of healing and likely some reviving.
This quest is difficult for the right reasons... it is a test of everybody's mettle, and your teamwork ability. All enemies are stronger than their normal GR26 counterparts, with Madoras at the end being exceptionally strong. A sound strategy is an absolute must, and somebody being a tank as well. For the Belcitane fight, your tank needs high RES, while for the others high DEF and/or AGI will do the trick. Divide up enemies if possible, and have a set plan of attack as the longer any fight goes, the worse things will get for your party. Having 1 or 2 clear out the respawning little guys is certainly helpful too, so they don't knock out favors or kill the victim they gang up on.
In the Belcitane fight, his attacks are magical while the pyro attacks are all physical. You need all forms of support here because belcitane's spells are aoe and may hit party members if they're close enough.
In the Ledom and Grazel fights, the grandies/lamias use mostly magical attacks, while the rival uses mostly physical attacks. You need all forms of support.
In the Madoras fight, while he is airborne all but 1 attack is magical, so focus on mana barrier and angels, but have fates on as well. When Madoras is on the ground, his attacks are 100% physical so only worry about fates and fortify.